An example of solid electrolytic capacitor is disclosed in Patent Document 1 described below. The prior art solid electrolytic capacitor includes a porous sintered body made of so-called “valve metal”. The sintered body is sealed in a resin package after a dielectric layer and a solid electrolyte layer are formed thereon.
Patent Document 1: JP-A 2003-163137
For instance, the solid electrolytic capacitor having the above-described structure is used as a bypass capacitor connected between an electronic device (such as a CPU) and a power supply circuit. In accordance with the recent speed increase and digitalization of electronic devices, a power supply system which operates stably and responds at high speed is demanded. Accordingly, also with respect to a solid electrolytic capacitor used for noise cancellation and the stabilization of a power supply system, excellent noise cancellation performance and high responsiveness in supplying power are demanded. Further, a large capacitance and high reliability for preventing firing are also demanded.
The capacitance of a solid electrolytic capacitor can be increased by increasing the surface area or volume of the porous sintered body. However, to merely increase the capacitance causes the degradation of frequency characteristics. Specifically, the frequency characteristics of a capacitor are generally determined by two factors of 1/ωCR and ωL. Herein, ω=2 Πf (f represents frequency), C represents capacitance, R represents resistance and L represents inductance. Of the two factors, the frequency characteristics of most solid electrolytic capacitors are substantially determined by 1/ωCR. Therefore, in doubling the capacitance, R need be cut in half to avoid the degradation of the frequency characteristics. Further, when the size of the porous sintered body is merely increased, the ESR (internal resistance, equivalent series resistance) increases. Therefore, in increasing the capacitance, the increase of ESR and the degradation of the frequency characteristics need be prevented. Particularly, when the thickness of the porous sintered body is increased to increase the size of the porous sintered body, the resistance in the electrical path from the obverse surface to the interior increases, which degrades the frequency characteristics. Further, the treatment liquid for forming a dielectric layer or a solid electrolyte layer in the porous sintered body becomes unlikely to permeate into the entire interior of the porous sintered body, so that the productivity of the solid electrolytic capacitor is degraded. Moreover, since a porous sintered body is made by sintering powder of niobium or tantalum, the reliability for preventing firing may be deteriorated by increasing the size of the porous sintered body.
Conventionally, therefore, to solve the above-described problems, the capacitance is increased by connecting a large number of small capacitors in parallel. However, such use of a large number of capacitors requires a large space for mounting the capacitors and also increases the manufacturing cost.
As means for increasing the capacitance without causing such disadvantages, to reduce the thickness of the porous sintered body maybe considered. When the thickness of a porous sintered body is reduced, the distance between electrodes is reduced. As a result, the impedance in the capacitor is reduced, which achieves low ESR. However, when the thickness of a porous sintered body is reduced, the length and the width are increased. Therefore, the possibility that the porous body warps in the sintering process or cracks increases. Moreover, even when the thickness of the porous sintered body is reduced, the heat generation in use is increased, because the entire volume is increased. Therefore, the performance of the capacitor itself may be reduced or the reliability for preventing firing may be degraded.